Manufacture of alloys containing dispersed fine particulate material

ABSTRACT

A method of preparing a metal alloy comprising a dispersion of particles of a first metallic material consisting of one or more refractory hard metals in a matrix of a second metallic material includes establishing a molten body of the second metallic material and introducing solid particles of the first metallic material into the molten body. Alternatively solid particles of a metalloid which reacts with the second metallic material to form particles of the first metallic material may be introduced into the molten body. The first metallic material has a higher melting point than the second metallic material and is substantially insoluble therein. The molten body is then stirred at a rate sufficient to effect shearing of the surfaces of the particles such that the surfaces are wetted by the molten body, and the molten body containing the resultant dispersion is cast in any desired manner.

FIELD OF INVENTION

[0001] The present application is directed to a novel method ofpreparing metal alloys containing finely divided particulate metal orintermetallic compounds as a substantially insoluble second phase. Aparticular embodiment is directed to manufacture of aluminum base masteralloys containing titanium together with boron or boron alone.

BACKGROUND TO THE INVENTION

[0002] Products containing dispersions of fine particulate non-metallicmaterial in metals and alloys, commonly known as metal matrixcomposites, are well known in metallurgical engineering. Examples aredispersions of metal oxides, nitrides, carbides and the like in a matrixof aluminum or alloys thereof. Commercial use of such composites wasdelayed for many years by the need to develop consistent and reliablemeans of manufacture of sound defect-free components. Mass production ofsuch components required melting and casting methods for conventionalmetal alloys to be adapted to metal matrix composites. This requiredcomplete wetting of the particle surfaces by the molten matrix metalwhich, in the early stages of this technology, was difficult to achieve.

[0003] A number of different methods of achieving such wetting have beenproposed. An early proposal by the International Nickel Company was tocoat the particulate with metallic nickel by exposure to gaseous nickelcarbonyl at an appropriate temperature. Metallic zinc and lithium andmagnesium oxide have also been proposed as coating materials.

[0004] A more successful approach has been the use of high shearstirring of molten metal containing solid non-metallic particulate usingspecially designed power-driven rotor systems to develop high relativevelocity between particulate dispersate and molten metal. Such methodshave been disclosed by Klier et al in U.S. Pat. No. 4,961,3461 and Skiboet al in U.S. Pat. No. 4,786,467. Klier et al disclose an uprighttapered cylindrical vessel containing a centrally located high speedrotor into which the molten matrix metal containing the particulatedispersate is introduced. The rotor shaft extends below said rotor tothe bottom of the vessel in which a second rotor is positioned. Thesecond rotor is conical in shape and positioned inside a fixed taperedwall. The narrow gap between the wall and the rotor serves as a zone ofhigh shear and can be varied at will by raising and lowering the rotor.Skibo et al disclose a circular section vessel into which a body of themolten matrix metal is introduced and into which the requiredparticulate dispersate is subsequently fed. The vessel contains aspecially designed centrally located dispersing impeller to effect highshear stirring. This is optionally augmented by a sweeping impellerlocated at the periphery of the vessel to sweep the particulate towithin the orbit of the dispersing impeller. This operation is carriedout under selected temperature and time conditions to promote wetting bythe molten metal of the exposed particulate surfaces.

[0005] A conventional method for manufacturing metal alloys with whichthe present invention concerned is to establish a stirred body of themolten metal into which is introduced a chemical compound of halides ordouble halides of the desired metallic additive(s) in powder form.Stirring is typically effected by electric induction. The compoundreacts with the molten metal to form the desired metallic additive whichon solidification and casting is present in particulate form within thecast product matrix. Examples of metal alloys of the present inventionare aluminum base master alloys containing titanium and boron. These arewidely used in aluminum and aluminum alloy manufacture by addition tomolten metal prior to casting to effect refinement of the ascast grainsize. The titanium and boron in such master alloys are typically presentas insoluble particles of titanium diboride, TiBsub2, suspended in themelt whilst titanium in excess of the stoichiometric proportions forformation of TiBsub2 (2.2:1) is in solution in the molten metal. Suchtitanium precipitates as the aluminide, TiAlsub3, on cooling andsolidification. A typical and commonly used formulation for such amaster alloy is aluminum 94%-titanium 5%-boron 1%.

[0006] A conventional method for manufacture of such master alloys is byaddition to a stirred body of unalloyed aluminum of a mixture ofpotassium titanium fluoride, Ksub2AIFsub6 and potassium borofluoride,KBFsub4 as powders premixed in the appropriate proportions to obtain thedesired master alloy formulation. Optionally, a portion of the titaniumin excess of the aforesaid stoichiometric requirements can be added assolid unalloyed titanium metal or as an aluminum titanium master alloy(e.g. containing 20% titanium). Such additions are made after completionof the mixed powder addition. The entire process is typically carriedout in a low or medium frequency electric induction furnace, theelectric power providing both heat and continuous stirring. Suchstirring is essential not only to effect uniform dispersion of thealloying ingredients but also to minimise gravity segregation of thetitanium diboride particles within the melt. The final stage of theprocess is casting the master alloy in an appropriate form for thedesired final product, e.g. an ingot for remelting or further workingdirectly to rod on a continuous casting machine.

[0007] It is desirable that casting take place as soon as possible afterthe alloying process to minimise both gravity segregation andagglomeration of the titanium diboride particles. The particles serve toeffect grain refinement when the master alloy is introduced into moltenaluminum or alloys thereof. For this to be efficiently carried out, itis essential that the size of the titanium diboride particles in themaster alloy be strictly controlled within given limits. For instance, amaster alloy user may require the particle size to be within the rangeof ¼ to 3 microns. Moreover, any coarse particles or agglomerates mayfunction as hard inclusions and impair the mechanical properties of thefinal product.

[0008] Aluminum-base master alloys containing boron alone are widelyused in the aluminum industry for aluminum conductor alloys requiringhigh electrical conductivity. Such conductivity may be impaired by smallquantities of the transition metals, titanium, chromium and vanadiumcommonly present in solution in commercial aluminum, rendering theproduct unsuitable for its intended use. Boron will combine with theaforesaid impurities to precipitate them as the borides thereof, inwhich form they have little or no effect on conductivity.

[0009] In the conventional manufacturing process of the master alloy,boron is introduced to molten aluminum in the form of potassiumborofluoride powder in a process analagous to that described above foraluminum titanium boron master alloy manufacture. Boron combines withthe aluminum to form the diboride, AlBsub2, or the dodecaboride,AlBsub12, depending on boron content and production conditions.Commercial master alloys typically contain either 3 or 4% boron.Particle size requirements are less stringent than for Al—Ti—B alloys.However, the borides must be fine enough to react completely with thetransition metals during the production time period available betweenaddition of the master alloy and casting of treated product. This can beparticularly critical when the master alloy is fed in rod form to thecasting trough of the treated product.

[0010] The above-described manufacturing methods for both AI—Ti—B andAl—B master alloys suffer many serious disadvantages. Firstly, use ofthe double fluorides of both titanium and boron entail severeenvironmental problems. The reaction with molten aluminum generatesvolatile fluorine-containing gases which must be contained and disposedof. This requires elaborate emission control installations which areexpensive to install and maintain and liable to malfunction. Thereaction products captured by the installation must also be disposed ofin an environmentally acceptable manner. Volumes of moltenfluoride-containing slags are also generated. Whilst these may have acommercial value, elaborate equipment is required for their separationfrom the molten alloy product and in either disposal or processing andpackaging. These processes increase labour requirements.

[0011] Particle size control, particularly in AI—Ti—B master alloyproduction requires close control of temperature and time conditions,not always successfully achieved in an industrial environment. Stringentmicroscopic examination may be required before packaging and shipment toensure compliance with the user's specification.

SUMMARY OF THE INVENTION

[0012] The present invention is based on the realisation that thevarious methods and apparatus for high shear stirring originallydeveloped and used for manufacture of metal matrix composites can alsoadvantageously be applied to manufacture of certain types of metalalloys. The invention provides a method of preparing a dispersion ofparticles of a first metallic material comprising one or more refractoryhard metals in a matrix of a second metallic material which comprisesthe metal or alloy matrix. The term “refractory hard metals” as usedherein is based on the characterisation provided in Schwarzkopf, Paul etal, Refractory Hard Metals, New York, Macmillan, 1953, Chapter I andincludes carbides, nitrides, borides and suicides of the transitionelements of the fourth to sixth groups of the periodic table. Thepresent invention also applies to borides of aluminum. According to oneaspect of the invention, a molten body of the second metallic materialis established into which solid particles of the first metallic materialare introduced. During this step, the molten body is stirred at a ratesufficient to establish relative shearing of the particle surfaces suchthat they become wetted by the molten metal. The resultant alloy is thencast by any desired method. The particulate first metallic material musthave a higher melting point than the second metallic material and besubstantially insoluble therein.

[0013] According to another aspect of the invention, a molten body ofthe second metallic material is established into which are introducedsolid particles of a metalloid which chemically reacts with the secondmetallic material to form particles of said first metallic material. Theterm “metalloid” is used here as defined in the McGraw-Hill Encyclopediaof Science and Technology, $′h Edition, Volume 11, 1997, page 67, namelyan element which exhibits the external characteristics of a metal butbehaves chemicallyboth as a metal and a non-metal. During this process,the molten body is stirred at a rate sufficient to establish shearing ofthe metalloid particle surfaces such that they become wetted by themolten metal thereby facilitating the reaction. The resultant alloy isthen cast by any desired method The particulate first metallic materialmust have a higher melting point than the second metallic material andbe substantially insoluble therein.

[0014] In both the above aspects of the invention, it is preferred thatthe molten body be established in an upright cylindrical vessel heatedby any desired means. Where fuel-fired heating is used, however, thecombustion products are preferably excluded from contact with the moltenmetal to avoid contamination or gas pick-up. Stirring of the moltenmetal body may be effected by a one or more power driven rotatingimpellers which serve to distribute the added particulate material andto effect shearing of the particulate surfaces and wetting thereof bythe molten metal.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0015] One embodiment of the present invention is directed to themanufacture of aluminum-base master alloys containing titanium and boronin which the boron is combined as the intermetallic compound TiBsub2.The method comprises establishing a molten body of unalloyed aluminum,into which particulate titanium diboride is introduced whilst effectinghigh speed stirring of said body. Shearing action of the molten aluminumon the titanium diboride particle surfaces is thereby effected which, asa result, become wetted by the aluminum. Titanium in excess of thestoichiometric requirements for TiBsub2 formation is also added inmetallic form, but there is no requirement for this to be as powder.Such titanium, for example, may be added as solid ingot or the like oras an aluminum-titanium master alloy.

[0016] Another embodiment of the invention is directed to themanufacture of aluminumbase master alloys containing boron present asone or both borides of aluminum, AlBsub2 and AlBsub12. In one aspect ofthis embodiment, a molten body of unalloyed aluminum is established intowhich particulate aluminum boride is introduced in the same way astitanium diboride as heretofore. In yet a further aspect, elementalboron is introduced. Boron is classified as a metalloid in theMcGraw-Hill Encyclopedia of Science and Technology, 8t′. Edition, Volume3, 1997, page 11. The molten body is stirred a using a high speed rotorto effect shearing and wetting of the boron particle surfaces. Achemical reaction of the boron with the aluminum is thereby promoted toform one or both borides of aluminum.

[0017] Both titanium diboride and elemental boron are available on themarket in particulate form. Whilst most commercial material is micronsized, sub-micron materaial is also available. The previously mentioneddisadvantages of the conventional methods of manufacture aresubstantially overcome by the present invention. Pollution problemsassociated with use of fluoride materials are eliminated resulting inmajor cost savings in emission control equipment. Particle size of themetallic alloying materials can be more closely controlled and fineraverage second phase particle sizes are feasible. No major quantities ofslag are generated other than the dross normally generated by meltingaluminum. In preferred embodiments of the invention, the foregoingprocesses can be operated either under vacuum or controlled inert gasatmosphere such as argon. These measures significantly improve alloycleanliness.

[0018] The advantages of the invention and further embodiments thereofwill now be readily apparent to a person skilled in the art, the scopeof the invention being defined in the appended claims.

1. A method of preparing a metal alloy comprising a dispersion ofparticles of a first metallic material consisting of one or morerefractory hard metals in a matrix of a second metallic materialcomprising the steps of: (i) establishing a molten body of said secondmetallic material (ii) introducing solid particles of said firstmetallic material into said molten body, whilst (iii) stirring saidmolten body at a rate sufficient to effect shearing of the surfaces ofsaid particles such that said surfaces are wetted by said molten body(iv) casting said molten body containing the resultant dispersion by anydesired means wherein said first metallic material has a higher meltingpoint than said second metallic material and is substantially insolubletherein.
 2. A method of preparing a metal alloy comprising a dispersionof particles of a first metallic material consisting of one or morerefractory hard metals in a matrix of a second metallic materialcomprising the steps of: (i) establishing a molten body of said secondmetallic material (ii) introducing into said body solid particles of ametalloid which reacts with said second metallic material to formparticles of said first metallic material (iii) stirring said moltenbody at a rate sufficient to effect shearing of the surfaces of theparticles of said metalloid such that said surfaces are wetted by saidmolten body (iv) casting said molten body containing the resultantdispersion by any desired means wherein said first metallic material hasa higher melting point than said second metallic material and issubstantially insoluble therein.
 3. A method according to claim 1wherein said molten body is established in an upright cylindrical vesselheated by any appropriate means and dispersion and wetting are achievedby one or more power driven impellers.
 4. A method according to claim 2wherein said molten body is established in an upright cylindrical vesselheated by any appropriate means and dispersion and wetting are achievedby one or more power driven impellers.
 5. A method according to claim 1wherein said second metallic material is unalloyed aluminum.
 6. A methodaccording to claim 2 wherein said second metallic material is unalloyedaluminum.
 7. A method according to claim 1 wherein said second metallicmaterial is unalloyed aluminum and said first metallic material istitanium diboride.
 8. A method according to claim 1 wherein said secondmetallic material is unalloyed aluminum and said first metallic materialis an aluminum boride,
 9. A method according to claim 2 wherein saidsecond metallic material is unalloyed aluminum and said metalloid iselemental boron.
 10. A method according to claim 1 wherein said moltenbody is maintained under vacuum.
 11. A method according to claim 2wherein said molten body is maintained under vacuum.
 12. A methodaccording to claim 1 wherein said molten body is maintained under anatmosphere of argon gas.
 13. A method according to claim 2 wherein saidmolten body is maintained under an atmosphere of argon gas.